Field of the Present Patent Application
The present patent application is generally directed to a transformer core comprising a plurality of amorphous metal strips. Specifically, the present patent application is generally directed to a method and apparatus for making an electric transformer core comprising a plurality of metallic strip packets or groups, each packet or group may comprise a plurality of thin amorphous metal strips. These thin strips of amorphous metal are arranged in a collection of packets or groups comprising multiple-strip lengths. These collections are then arranged to surround a window of a core of the transformer where the window of the core first resides on a winder. However, aspects of the present application may be equally applicable in other scenarios as well.
Description of Related Art
Electrical-power transformers are used extensively in various electrical and electronic applications. For example, transformers transfer electric energy from one circuit to another circuit through magnetic induction. Transformers are also utilized to step electrical voltages up or down, to couple signal energy from one stage to another, and to match the impedances of interconnected electrical or electronic components. Transformers may also be used to sense current, and to power electronic trip units for circuit interrupters. Still further, transformers may also be employed in solenoid-equipped magnetic circuits, and in electric motors.
A typical transformer includes two or more multi-turned coils of wire commonly referred to as “phase windings.” The phase windings are placed in close proximity so that the magnetic fields generated by each winding are coupled when the transformer is energized. Most transformers have a primary winding and a secondary winding. The output voltage of a transformer can be increased or decreased by varying the number of turns in the primary winding in relation to the number of turns in the secondary winding.
The magnetic field generated by the current passing through the primary winding is typically concentrated by winding the primary and secondary coils on a core of magnetic material. This arrangement increases the level of induction in the primary and secondary windings so that the windings can be formed from a smaller number of turns while still maintaining a given level of magnetic-flux. In addition, the use of a magnetic core having a continuous magnetic path helps to ensure that virtually all of the magnetic field established by the current in the primary winding is induced in the secondary winding. An alternating current flows through the primary winding when an alternating voltage is applied to the winding. The value of this current is limited by the level of induction in the winding.
The current produces an alternating magnetomotive force that, in turn, creates an alternating magnetic flux. The magnetic flux is constrained within the core of the transformer and induces a voltage across in the secondary winding. This voltage produces an alternating current when the secondary winding is connected to an electrical load. The load current in the secondary winding produces its own magnetomotive force that, in turn, creates a further alternating flux that is magnetically coupled to the primary winding. A load current then flows in the primary winding. This current is of sufficient magnitude to balance the magnetomotive force produced by the secondary load current. Thus, the primary winding carries both magnetizing and load currents, the secondary winding carries a load current, and the core carries only the flux produced by the magnetizing current.
Certain modern transformers generally operate with a high degree of efficiency. Magnetic devices such as transformers, however, undergo certain losses because some portion of the input energy to the transformer is inevitably converted into unwanted losses such as heat. A most obvious type of unwanted heat generation is ohmic heating—heating that occurs in the phase windings due to the resistance of the windings.
Traditionally, electrical transformer cores have been formed completely of high grain oriented silicon steel laminations. Over the years, improvements have been made in such high grained oriented steels to permit reductions in transformer core sizes, manufacturing costs and the losses introduced into an electrical distribution system by the transformer core. As the cost of electrical energy continues to rise, reductions in core loss have become an increasingly important design consideration in all sizes of electrical transformers.
In order to reduce these undesired affects of such high grain oriented steel type transformers, amorphous metals having a non-crystalline structure have been used in forming elecromagnetic devices, such as cores for electrical transformers. Generally, amorphous metals have been used because of their superior electrical characteristic relative to high grain oriented silicon steel laminations. For this reason, amorphous ferromagnetic materials are being used more frequently as transformer base core materials in order to achieve a decrease in transformer core operating losses.
Generally, amorphous metals may be characterized by a virtual absence of a periodic repeating structure on the atomic level, i.e., the crystal lattice. The non-crystalline amorphous structure is produced by rapidly cooling a molten alloy of appropriate composition such as those described by Chen et al., in U.S. Pat. No. 3,856,513, herein incorporated by reference and to which the reader is directed for further information. Due to the rapid cooling rates, the alloy does not form in the crystalline state. Rather, the alloy assumes a metastable non-crystalline structure representative of the liquid phase from which the alloy was formed. Due to the absence of crystalline atomic structure, amorphous alloys are frequently referred in certain literature and elsewhere as “glassy” alloys.
Due to the nature of the manufacturing process, an amorphous ferromagnetic strip suitable for winding a distribution transformer core, for example, is extremely thin. For example, the thickness of a typical amorphous metallic strip may nominally be on the order of 0.025 mm versus a thickness of approximately 0.250 mm for typical grain oriented silicon steel. Moreover, such amorphous metallic strips are quite brittle and are therefore easily damaged or fractured during the processing and handling of such strips. For example, a typical amorphous metallic strip may nominally. Consequently, the handling, processing, and fabrication of wound amorphous metal cores presents certain unique manufacturing challenges of handling the very thin strips. This is particularly present throughout the various manufacturing steps of winding the core, cutting and rearranging the core laminations into a desired joint pattern, shaping and annealing the core, and finally lacing the core through the window of a preformed transformer coil. Of particular importance is the lacing step which must be effected with heightened care so as to avoid permanently deforming the core from its annealed configuration after the core has been laced into the coil window. That is, if the core is not exactly returned to its annealed shape, stresses are introduced during the lacing procedure. Consequently, if there are significant stresses remaining after lacing, the potential low core loss characteristic offered by the amorphous metal core material is not achieved. Since amorphous metal laminations are quite weak and have little resiliency, they are readily disoriented during the lacing step, resulting in permanent core deformation if not corrected. In addition to this concern, there is also a potential concern that the lacing step is carried out with sufficient care such as to avoid fracturing the brittle amorphous metal laminations.
However, the relatively thin strips ribbons of amorphous metals present certain core manufacturing challenges during the handing, processing, assembly and annealing of such amorphous metal transform cores. As just one example, certain amorphous metal transformer cores generally require a greater number of laminations or groupings or stacks of strips in order to form a desired amorphous metal core. As such, amorphous metal cores comprising a larger number of laminations tend to present certain difficulties and challenges in handling during the various processing steps that may be involved as the plurality of metallic strip groupings and collections are eventually processed, sheared, and then formed into an amorphous metal core.
In addition, the magnetic properties of the amorphous metals have been found to be deleteriously affected by mechanical stresses such as those created by the fabricating steps of winding and forming the amorphous metal groupings and stacks into a desired core shape.
Certain known methods and/or systems for manufacturing amorphous metal transformer cores are known have attempted to solve or reduce these known manufacturing challenges. As just one example, U.S. Pat. No. 5,285,565 entitled “Method for Making a Transformer Core Comprising Amorphous Steel Strips Surrounding The Core Window” herein entirely incorporated by reference and to which the reader is directed, teaches such a method and system for making a transformer core comprising a plurality of groupings of amorphous metal strips. As described in U.S. Pat. No. 5,285,565, the disclosed method utilizes a plurality of spools of amorphous steel strip in each of which the strip is wound in a single-layer thickness. For example, and as illustrated in FIG. 1 of U.S. Pat. No. 5,285,565, a pre-spooler comprising five starting spools is illustrated. As the inventors describe in this patent, the strip from the five starting spools must first be unwound and then re-wound onto the pre-spooler. In this manner, the five single ply spools are unwound so as to create a five (5) ply ribbon or strip that then must be wound onto the pre-spooler.
During a subsequent processing step, by way of a pre-spooling machine, the single-layer thickness amorphous metal strips from the five starting spools are unwound. In a subsequent processing step, these single-layer thickness strips are then combined to form a strip of multiple-layer thickness (a five ply composite strip) that is then wound onto a plurality of master reels, on each of which the strip is wound in multiple-layer thickness. These master reels comprising the amorphous metal strips of multiple-layer thickness are then placed on a plurality of payoff reels.
In a next process step, these various multiple-layer thickness strips are unwound from these payoff reels and then combined into a final composite metallic strip. This final composite metallic strip would then comprise an overall thickness in strip layers equal to the sum of the strip layers in the combined multiple-layer thickness strips. Finally, the composite strip is cut into a plurality of groupings or packets, or lengths of composite strip. These plurality of groupings or packets are then constructed onto a hollow core, which form has a window about which the various cut sections are wrapped.
Although the pre-spooler and master spool system and methods disclosed in U.S. Pat. No. 5,285,565 purports to provide certain advantages over other known methods of amorphous metal transformer core manufacturing, there are a number of perceived disadvantages of utilizing such a system comprising one or more master spools or multiple-ply coils. For example, with such a system comprising a plurality of multiple-ply coils, each single coil must first be mounted onto an uncoiler and then single-ply strip must be unwound and then fed into the pre-spooler in a controller and uniform manner. As such, there is an associated set up cost, labor cost and machine cost associated with first mounting and then unwinding five single sheet spools and then rewinding them back into a 5-ply spool.
In addition, there is an associated additional machine cost since an amorphous transformer core manufacturer is required to purchase, install, and maintain not only a pre-spooler and a master-spooler but also a separate apparatus that combines the multiple-layer thickness strips unwound from the plurality of master spools. As such, addition manufacturing floor space must be allocated not only to the machine for pre-spooling but also for the overall assembly apparatus for fabricating the transformer core itself.
In addition, with the multiple-ply coil system described above, each of the five individual amorphous metal strips within the five-ply group will wrap up around the spool at a slightly different diameter. That is, with the five-ply metal strip grouping, the outer or top most metallic strip will be slightly longer than the inner or bottom most metallic strip since the outer or top most strip most wrap around the spool at a slightly larger spool diameter. As such, each of the various metal strips wound around a multiple-ply coil will comprise different lengths. Therefore, after running a number of laps off the five-ply coil during assembly of the transformer core (such as the five-ply coil illustrated in U.S. Pat. No. 5,285,565), an operator of the overall system must first stop the entire line since eventually one of the outer most strips within a five-ply coil will eventually be longer than the other strips within the grouping. After stopping the machine, the operator must then somehow remove the extra material from the longer of the five strips so as to even these lengths up so that all of the strips of the multi-ply coil comprise the same overall length. As those of skill in the art will recognize, oftentimes, the machine operator will either cut or tear this “extra” amorphous strip material from longer strip so that all of the sheets will comprise the same length. Repeatedly stopping, removing the excess amorphous strip material, and starting the overall system back up again increases overall manufacture costs by increasing overall system down time and driving up overall labor costs per pound of the to be manufactured transformer cores. In addition, in the prior art apparatus as illustrated in U.S. Pat. No. 5,285,565, an operator would have to remove this excess amorphous strip material from not just one multi-ply coil but from a total of four multi-ply coils since they would all unwind uniformly. Moreover, constant starting and stopping these heavy duty pre-spooling and spooling machines also increases the overall wear and tear on the machinery.
In addition, after having to repeatedly stop and then restart the overall combining apparatus as illustrated in U.S. Pat. No. 5,285,565, the machine operator must then, at the various points of the longest metallic strips cut or tear the amorphous strips, and then somehow re-connect the torn strip materials. Again, for a combining apparatus as illustrated in this prior art patent, an operator must cut or tear at least four amorphous strips. Then, the operator must apply some type of adhesive or connecting mechanisms (e.g., such as a high temperature resistant tape) so as to hold the loose or torn amorphous metal strips back together. This of course adds further costs to the overall manufacturing process while also driving up overall processing and manufacturing times. In addition, placing the adhesive or connecting mechanism (such as tape) can cause further manufacturing challenges downstream of the uncoilers when running a composite metallic strip comprising a plurality of these thin metallic strips at relatively high speeds.
In addition, certain high temperature resistant tapes that are typically used in this assembly process can cause further complications during subsequent process steps of the amorphous metallic cores. As just one example, one high temperature resistant tape this is typically used to hold these torn amorphous metallic strips together is Kapton tape. As those of skill in the art will recognize, one advantage to using Kapton tape to hold these loose metallic strips together is that this high temperature tape is generally known to remain relatively stable even when used in a wide range of temperatures. For example, Kapton tape tends to remain stable if it is heated from about −273 to about +400 degrees Celsius.
However, use of such a high temperature tape to reconnect the amorphous metal strips presents certain problems during transformer core manufacturing. First, Kapton tape is quite expensive and therefore use of such tape increases the overall cost of manufacturing. In addition, and as discussed above, because of its stability in a wide rage of temperatures, Kapton tape is resistant to burning at temperatures used during the transformer annealing process, typically on the order of 330 to 470 degrees Celsius. Because of its resistance to burning during the transformer core annealing process, the Kapton tape can cause certain problems during the transformer annealing process.
Certain other tapes that do not resist burning at transformer core annealing temperatures can leave a residue from the burned tape in the transformer core. Such tape residue can cause other problems. For example, in one worst case scenario, such tape residue can react with the transformer oil. As another example, after the transformer core annealing step, certain tapes may result in a residue that can stain the strips in the transformer core and possibly cause rust in the core.
Accordingly, Applicants' presently proposed method and apparatus is directed to manufacturing and providing an amorphous metal transformer core that is cost effective to manufacture, that has low energy losses, and that is energy efficient. Applicants' proposed method and apparatus is also directed to an amorphous metal transformer core in which the difficulties of handling and processing the amorphous metal strips to perform the manipulative steps of the fabrication process are reduced and the mechanical stresses induced into the amorphous metal strips and hence the core during its fabrication process are reduced. In addition, in Applicants presently disclosed systems and methods, fabrication of the amorphous metal core process is simplified since it does not require a pre-spooling step and therefore a costly pre-spooling machine and corresponding maintenance and manufacturing floor space for placement of such a machine.
In addition, Applicants' presently disclosed system and method reduces the overall time for fabricating a desired amorphous metal transformer core. Moreover, with Applicants' presently disclosed system and method reduces the amount of scrap metallic strip material generated during manufacturing since the system operator no longer needs to stop the entire process so as to remove a portion of the multi-ply strip groupings so as to even out the metallic strips of unequal length and then reconnect the metallic strip. As such, there is no longer a need to use a high temperature tape or other type of connection mechanism so as to connect the loose strip ends of the amorphous material. These and other objects of the Applicants disclosed systems and method will become apparent to those skilled in the art upon consideration of the following illustrations and detailed description.